A boulder booty is a butt that is typically big, which in most cases it would be considered sexy, but it isn't in this scenario. A boulder booty has no particular shape and it looks as if when god was making this girl's butt he was just slapping on layers of fat like Play-Doh. Most boulder booties consist of cellulite, wrinkles, dimples, and other things that would make it nasty looking. This is where the phrase, "Just because its a big booty doesn't mean its a nice booty" comes from. Women that have a boulder booty ALWAYS have a FUPA.